Golf balls are generally divided into two classes: solid and wound. Solid golf balls include a solid core of one or more layers, a cover of one or more layers, and optionally one or more intermediate layers. Wound golf balls typically include a solid, hollow, or fluid-filled center, surrounded by tensioned elastomeric material, and a cover. Solid golf balls, as compared with wound balls, are more durable and resilient, providing better distance than wound balls due to their higher initial velocity upon impact with a club face. Meanwhile, the wound construction provides a softer “feel”, lower compression and higher spin rate—characteristics often preferred by accomplished golfers who are able to control the ball's flight and positioning.
By altering solid golf ball construction and composition, manufacturers can vary a wide range of playing characteristics such as resilience, durability, spin, and “feel”, optimizing each according to various playing abilities and achieving a solid golf ball possessing feel characteristics more like their wound predecessors. For example, by shifting the density (the weight or mass of the golf ball) toward the center of the ball, the moment of inertia of the golf ball can be reduced, thereby increasing the initial spin rate of the ball as it leaves the golf club head as a result of the higher resistance from the golf ball's moment of inertia. In this regard, core is the “engine” of the golf ball when hit with a club head. That is, it is the spring of the ball and its principal source of resiliency. Meanwhile, the intermediate layers based on ionomers aid in maintaining initial speed, contribute to desired spin rate, and improve playability/impact durability as well as acting as a moisture barrier to protect the cores from the COR loss. The cover, while originally intended to protect the golf ball from scuffing, may also be modified to target a desired spin rate, feel, and playability, even addressing such issues as “lift” and “drag”. Golf ball manufacturers have sought to incorporate and configure materials in the core, intermediate layer and/or cover in order improve performance and achieve desired characteristics. In this regard, multi-layered cover configurations employing a very thin thermoplastic outer cover layer and a much softer thermoset inner cover layer improve durability, resilience and provide a unique spin profile. See e.g., U.S. 2002/0147057 A1 of Binette et al. Heretofore however, such golf balls have been manufactured via conventional compression or injection molding processes—i.e., molding an inner cover layer over the core and molding the outer cover layer over the inner cover layer. Id., infra. These processes are not well-suited for or adapted for constructing golf balls incorporating very thin cover layers, as quality control issues arise including poor concentricity, matability and/or contouring between inner and outer cover layers, as well as lack of consistency in cover layer wall thickness.
Retractable pin injection molding (RPIM) or compression molding methods are the most common methods for molding thermoplastic cover layers around solid cores. Both processes require heat and pressure to form the cover, the pressure required can cause deformation of the core. During the process of compression molding thin covers, the core is severly deformed from the pressure exerted by the melting polymer as the mold closes, causing a “blow out” of the core. Other issues can be roundness of the ball, core shifting, and cover concentricity. Similar issues occur when trying to mold thin covers using the RPIM process resulting from the “pinching” of the core by the retractable pins that center the core as the molten plastic is injected into the mold under high pressure. Similarly, the issues are “blow out” (exposed cores), core shifting, out of roundness, and cover concentricity.
Accordingly, there is a need for a golf ball manufacturing process which addresses and resolves these manufacturing issues.